This application is based on Japanese Patent Application No. 11-290270 (1999) filed Oct. 12, 1999, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to an ink jet printing apparatus that prints on a print medium and more specifically to an ink jet printing apparatus capable of changing the amount of ink to be ejected.
2. Description of the Related Art
With the widespread use of office automation equipment such as personal computers and word processors in recent years, the use of printing apparatus, the peripheral devices of the equipment, is also spreading quickly. Among the printing systems of the printing apparatus there are a wire-dot system, a heat transfer system, and an ink jet system. These printing systems have their own print heads that perform a predetermined printing on a print sheet being fed.
There are growing demands on these printing apparatus for faster printing speed, higher resolution, higher image quality and lower noise. With the speeds of MPU""s increasing and the integration levels of semiconductor memories becoming higher in recent years, the occasion of handling image data has increased. Hence, demands are increasingly growing for a high quality printing of images with half tone.
Among the methods for representing a halftone in the ink jet printing apparatus are a method which controls the number of dots applied to a unit area of a print medium (a pseudo halftone representation using binary values) and a method which uses a plurality of print heads with different ink ejection amounts or with different densities of ink and selects and activates an appropriate head according to a halftone to be printed.
However, there are the following problems with the conventional halftone representation methods.
With the pseudo halftone representation method using binary values, because a single pixel is represented with a plurality of dots, the resolution is degraded.
With the method of representing a halftone by using a plurality of heads with different ink ejection amounts or with different ink densities, the size reduction and space reduction are hindered, with additional problem of an increased cost. In a multivalued full color printing apparatus, in particular, it is necessary to use a plurality of heads for each color, making the realization of this halftone representation method very difficult.
Japanese Patent Application Laying-Open No. 55-132259 (1980) proposes a method which provides two electrothermal transducers (of the same or different sizes) in each nozzle to change a dot size and thereby realize a wide grayscale range and a high image quality with a very simple construction. This constitutes a very important technology in performing a multivalued printing. Provision of a plurality of electrothermal transducers in each nozzle, however, requires setting the nozzle size relatively large, giving rise to a problem that the resolution cannot be enhanced easily.
In the method that provides a plurality of electrothermal transducers in each nozzle and ejects different amounts of ink from each nozzle, the ejection speed is higher when the amount of ink ejected is large than when it is small. This causes variations in the ejection speed, which in turn causes deviations in the dot landing positions, resulting in a disturbed printed image.
It is also proposed that, to eject a small amount of ink a bubble produced by the activation of the electrothermal transducer is not brought into communication with an external air and that only when a large amount of ink is to be ejected, is the bubble brought into communication with an external air. For that purpose, a method has been proposed and implemented which gives one drive pulse for the ejection of one droplet when a small amount of ink is to be ejected and which gives a plurality of pulses for the ejection of one droplet when a large amount of ink is to be ejected. In this case, too, the above-described problem occurs, i.e., the ejection speed is higher when a large amount of ink is ejected than when a small amount is ejected, causing variations in the ink ejection speed and therefore deviations in the dot landing positions, which in turn disturbs an image.
In both of the above two methods, when the ink ejection amount is changed to form a large dot, followed by a small one, a large one, a large one, a small one, and so on for each pixel, meniscus vibrations occur, which in turn causes positional deviations of the meniscus in the nozzle portion. When the meniscus is tilted due to the positional deviation, the dot landing precision deteriorates significantly. Variations in the ink ejection amount should be kept to within xc2x110% but depending on driving conditions they are found to be as large as about xc2x120%. In the construction in which a plurality of electrothermal transducers are provided in each nozzle, if it is assumed that the nozzle that ejects a large ink droplet has twice the ink ejection amount of a nozzle that ejects a small ink droplet, the refill frequency of the large droplet ejecting nozzle is reduced to one-half that of the small droplet ejecting nozzle. The print speed of the ink jet printing apparatus therefore is limited by the refill frequency for the large droplet ejecting nozzle, making it impossible to obtain a sufficiently fast print speed.
The present invention has been accomplished to solve the above-described conventional problems. It is therefore an object of the invention to provide an ink jet printing apparatus and an ink cartridge which can form a multivalued image by changing the ink ejection amount from each nozzle, easily enhance the resolution of the image, reduce ink ejection speed variations and meniscus vibrations when forming large or small dots, and produce high quality images, without using a plurality of electrothermal transducers in each nozzle or a plurality of print heads for ejecting ink droplets of the same color with different densities.
In the first aspect of the present invention, there is provided an ink jet printing apparatus comprising:
a print head having electrothermal transducers in ink ejection nozzles; and
drive control means for generating a drive pulse for controlling activation of the electrothermal transducers in accordance with print data;
wherein the drive pulse causes the electrothermal transducers to generate thermal energy to eject ink droplets from the nozzles onto a print medium to print an image;
wherein the drive control means changes a drive voltage and a drive pulse width simultaneously for the print head in accordance with the print data.
In the second aspect of the present invention, there is provided an ink jet printing apparatus comprising:
a print head having electrothermal transducers in ink ejection nozzles; and
drive control means for generating a drive pulse for controlling activation of the electrothermal transducers in accordance with print data;
wherein the drive pulse from the drive control means causes the electrothermal transducers to generate thermal energy to eject ink droplets from the nozzles onto a print medium to print an image;
wherein the drive control means, when ejecting small droplets, increases a drive voltage for the nozzles and shortens the drive pulse width for the nozzles and, when ejecting large droplets, reduces the drive voltage and relatively elongates the drive pulse width.
In the third aspect of the present invention, in an ink jet printing method of performing printing an image by utilizing a print head having electrothermal transducers in ink ejection nozzles, generating a drive pulse for controlling activation of the electrothermal transducers, supplying the drive pulse to the electrothermal transducers, and then causing the electrothermal transducers to generate thermal energy to eject ink droplets from the nozzles onto a print medium;
wherein a drive voltage and a drive pulse width for the print head are simultaneously changed in accordance with a print data.
In the fourth aspect of the present invention, in an ink jet printing method of performing printing an image by utilizing a print head having electrothermal transducers in ink ejection nozzles, generating a drive pulse for controlling activation of the electrothermal transducers, supplying the drive pulse to the electrothermal transducers, and then causing the electrothermal transducers to generate thermal energy to eject ink droplets from the nozzles onto a print medium;
wherein when ejecting small droplets, a drive voltage for the electrothermal transducers is increased and a drive pulse width for the electrothermal transducers is shortened; and
when ejecting large droplets, the drive voltage is reduced and the drive pulse width is relatively elongated.
In the fifth aspect of the present invention, ink jet print head having a protective film deposited over the electrothermal transducers arranged on a substrate, the protective film being 6,000 xc3x85 or less thick.
As described above, in this invention a desired amount of ink can be ejected stably by changing the drive pulse width and the drive voltage. That is, the size of the bubble that changes the amount of ink ejected is determined by the pulse width and the drive voltage of the drive pulse for the electrothermal transducers in the print head. Controlling both of the pulse width and the drive voltage can control the ink ejection in the same head.
For example, when comparison is made between a configuration with a high drive voltage and a short drive pulse width and a configuration with a low drive voltage and a long drive pulse width, the time it takes for the former configuration to transmit heat from the electrothermal transducer to the liquid such as ink is shorter and therefore the amount of ink ejected is smaller than that of the latter configuration. This is because the former configuration has a thinner ink layer (high temperature layer), heated at high temperature, that contributes to a bubble generation.
Hence, to reduce the ink ejection amount, the drive pulse is set to have a higher voltage and a shorter pulse width; and to increase the ink ejection amount, the drive pulse is set to have a lower voltage and a large pulse width. The inventor of this invention measured the size of the bubble formed on the electrothermal transducer and it has been confirmed that the generated bubble is apparently smaller when the drive pulse is set to have a higher voltage and a shorter pulse width. The measurements were made under the condition that the injected energy was constant. That is, the drive voltage was set so that the energy injected into the electrothermal transducer would not change depending on the size of the pulse width. By changing the drive voltage and the drive pulse width simultaneously in this manner, the bubble generating force in the ink jet print head can be controlled, making it possible to change the ink ejection amount by using the same electrothermal transducer.
The bubble generating force is changed by changing the drive voltage and the drive pulse width simultaneously. When a small ink droplet is to be ejected, it is done by keeping the bubble from coming into communication with the external air outside the nozzle. When a large ink droplet is to be ejected, the bubble is communicated with the external air. This can change the ink ejection amount in a wider range.
Further, voltage supply paths for supplying a plurality of different drive voltages are formed in the print head, and the voltage supply paths are disconnected or connected to change the voltage and width of the drive pulse supplied to the electrothermal transducer. This makes it possible to quickly change the drive pulse for the electrothermal transducer, allowing the ink ejection amount to be changed for each pixel.